The invention relates to metal/air batteries, and particularly such batteries having recirculating electrolyte.
Metal/air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having opposite surfaces respectively exposed to the atmosphere and to the aqueous electrolyte of the cell. During cell operation oxygen is reduced within the cathode while metal of the anode is oxidized, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuitry can be connected. Present-day commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) in association with a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, zinc, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight, and ability to function as anodes in metal/air battery using a variety of electrolytes.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface.
Aqueous electrolytes for metal-air batteries consist of two basic types, namely a neutral-pH electrolyte and a highly alkaline electrolyte. The neutral-pH electrolyte usually contains halide salts and, because of its relatively low electrical conductivity and the virtual insolubility of aluminum therein, is used for relatively low power applications. The highly alkaline electrolyte usually consists of NaOH or KOH solution, and yields a higher cell voltage than the neutral electrolyte.
In neutral-pH electrolyte, the cell discharge reaction may be written: EQU 4Al+3O .sub.2 +6H.sub.2 O.fwdarw.4Al(OH).sub.3 (solid)
In alkaline electrolyte, the cell discharge reaction may be written: EQU 4Al+3O.sub.3 +6H.sub.2 O+4KOH.fwdarw.4Al(OH).sub.4.sup.- +K.sup.+ (liquid solution),
followed, after the dissolved potassium (or sodium) aluminate exceeds saturation level, by: EQU 4Al(OH).sub.4.sup.- +4K.sup.+ .fwdarw.4Al(OH).sub.3 (solid)+4KOH
In addition to the above oxygen-reducing reactions, there is also an undesirable, non-beneficial reaction of aluminum in both types of electrolyte to form hydrogen, as follows: EQU 2Al+6H.sub.2 O.fwdarw.2Al(OH).sub.3 +3H.sub.2 (gas)
There is a need for a metal-air battery which can be used as an emergency power source at locations where electric supply lines do not exist. Such a battery must have a high energy capacity and a high power density and be capable of running for a long period of time under high load. When the battery is run under high load, not only do large amounts of aluminum hydroxide accumulate in the electrolyte, but quantitites of hydrogen also form from the surface of the electrolyte. As with other batteries this hydrogen can easily reach explosive concentrations. Considerable heat is also evolved, resulting is evaporative electrolyte loss.
A battery intended as an emergency power supply is described in U.S. Pat. No. 4,490,443, issued Dec. 25, 1984. That battery uses a plurality of individual metal air cells with an electrolyte recirculated through the cells by means of a centrifugal or impeller pump. It is capable of operating under high load, but it has major disadvantages in that the individual cells are not easily exchanged when the anode is depleted, it does not provide a means for managing accumulated solids, the entire battery is dependent on a single pump and it does not provide a satisfactory means for avoiding hydrogen build-up in the electrolyte reservoir.
It is an object of the present invention to develop a battery capable of long-time operation under high load which does not have the above disadvantages.